1. Field of the Invention
The present invention relates to a surface acoustic wave filter and surface acoustic wave filter apparatus used in an optical communication device and mobile radio communication device.
2. Description of the Prior Art
A surface acoustic wave filter (to be referred to as SAW (Surface Acoustic Wave) filter hereinafter) has features such as compactness, high stability, high precision, high reliability, and no need for adjustment, and is widely used as a functional element for controlling an RF signal in an optical communication device and mobile communication device. For example, the SAW filter is used as a functional element such as a frequency selection element, filter element, resonator, and delay element.
The SAW filter controls an electrical RF signal through a surface acoustic wave propagating on the surface of the base material. As the base material, a piezoelectric material such as rock crystal, LBO, lithium tantalate (LiTaO3), or lithium niobate (LiNbO3) is generally used. In the following description, a base material made of a piezoelectric material is called a chip substrate. A plurality of come electrode patterns (Interdigital Transducers: IDTs) are formed on the chip substrate by using, e.g., the thin film formation technique and photolithography technique.
In the SAW filter, a pair of comb electrode patterns, or several pairs of comb electrode patterns arranged in the propagating direction of the surface acoustic wave are formed. The SAW filter can select a frequency which depends on the line width of the comb electrode patterns by converting an electrical RF signal into a surface acoustic wave and vice versa on a surface wave propagation path serving as a range where the comb electrode patterns are formed.
The SAW filter is generally used as it is accommodated in a package. In the following description, a SAW filter accommodated in a package is called a SAW filter apparatus. The comb electrode patterns are connected, through bonding wires or the like, to an input signal electrode terminal, output signal electrode terminal, and ground terminals formed on a package which is to accommodate the SAW filter.
The piezoelectric material has pyroelectric properties. In mounting a SAW filter apparatus on a printed wiring board or the like, when soldering is performed by reflow, or when a thermal shock test is performed, causing a sharp temperature change in the surrounding environment, polarization may occur in the piezoelectric material to generate charges on the surface of the chip substrate. When charges are generated on the surface of the chip substrate, they produce an electric field on the comb electrode patterns. When the electric field of a portion where the charges are concentrated most reaches a dielectric breakdown level, discharge occurs between the comb electrode patterns. In the worst case, pattern destruction is caused by discharge.
In order to prevent discharge between the comb electrode patterns, a package made of a ceramic material is used as the package for accommodating the SAW filter, and a metal film is formed on the bottom surface of the interior of the package by plating or vapor deposition. More specifically, a ceramic package, in which a metal film comes into contact with the lower surface of the chip substrate of the SAW filter when the SAW filter is accommodated in it, is used. Then, the ground electrode of the SAW filter and the metal film are connected to each other. When this package is used, even if polarization occurs to generate charges, the generated charges are released to the metal film, and destruction of the comb electrode patterns is prevented.
A plastic package is sometimes used in order to reduce the manufacturing cost of the SAW filter. It is, however, difficult to form a metal film on the plastic package by plating or vapor deposition. In other words, it is difficult to provide a package with a means for releasing charges which are generated when polarization occurs. Accordingly, a SAW filter cannot but be packaged with no metal film being present between the plastic package and SAW filter. As a result, to be able to use a plastic package, it is important that the chip substrate itself of the SAW filter have a structure with which polarization does not occur or which releases charges generated when polarization occurs.
The present invention has been made in consideration of the above situation in the prior art, and has as its object to provide a SAW filter with a structure that can prevent the comb electrode patterns from being destroyed by charges generated when polarization occurs, and a SAW filter apparatus with a structure that can prevent the comb electrode patterns from being destroyed by charges even if no countermeasure is taken on the package side.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a surface acoustic wave filter in which a plurality of pairs of comb electrode patterns are formed on an upper surface of a chip substrate made of a piezoelectric material, comprising a high-resistance pattern formed to surround a peripheral portion of the chip substrate on an upper surface side, and a plurality of patterns formed to connect the high-resistance pattern to the respective comb electrode patterns.
In order to achieve the above object, according to the second aspect of the present invention, there is provided a surface acoustic wave filter in which a plurality of pairs of comb electrode patterns are formed on an upper surface of a chip substrate made of a piezoelectric material, comprising a first pattern formed on the upper surface of the chip substrate so as to extend from part of at least one member of each of the pairs of comb electrode patterns, a second pattern formed on the upper surface of the chip substrate so as to oppose the first pattern at a predetermined gap, and a dummy electrode pattern formed on the upper surface of the chip substrate so as to be adjacent to the second pattern and connected thereto.
In order to achieve the above object, according to the third aspect of the present invention, there is provided a surface acoustic wave filter in which a plurality of pairs of comb electrode patterns are formed on an upper surface of a chip substrate made of a piezoelectric material, wherein opposing portions of two comb electrode patterns that form a pair are not sharp.
A surface acoustic wave filter apparatus according to the present invention is obtained by accommodating either one of the surface acoustic wave filters according to the first to third aspects in a plastic package.
Furthermore, a metal plate-like portion is formed at a center of the package that accommodates the surface acoustic wave filter, and a lower surface of the chip substrate which constitutes the surface acoustic wave filter is fixed to the package with a conductive adhesive.
As is apparent from the above aspects, according to the present invention, the SAW filter has the high-resistance pattern to surround the peripheral portion of the chip substrate on the upper surface side, and the patterns to connect the respective comb electrode patterns to the high-resistance pattern. The comb electrode patterns are set at the same potential, and accordingly discharge between the comb electrode patterns can be prevented, so destruction of the comb electrode patterns can be prevented.
When the first pattern formed by extending part of the comb electrode pattern, the second pattern formed to oppose the first pattern at the predetermined gap, and the dummy electrode pattern to be connected to the second pattern are formed on the upper surface of the chip substrate, even if polarization occurs in the chip substrate to generate charges, thus causing discharge, this discharge occurs in the gap, so discharge between the comb electrode patterns can be prevented.
Of the respective comb electrode patterns, when the opposing portions of two comb electrode patterns that form a pair are formed to be not sharp, even if polarization occurs in the chip substrate to generate charges, discharge does not occur easily between the comb electrode patterns.
If such a SAW filter is accommodated in a package and the resultant structure is used as a SAW filter apparatus, particularly when the SAW filter apparatus is mounted on a substrate or the SAW filter apparatus is tested, destruction of the comb electrode patterns, which is caused when charges are generated in the chip substrate due to polarization, can be prevented.
When the metal plate-like portion is formed on the package, if the lower surface of the chip substrate is fixed to the package with the conductive adhesive, the effect of preventing destruction of the comb electrode patterns, which is caused when charges are generated in the chip substrate due to polarization, becomes larger.